1. Field of the Invention
This invention generally relates to an image signal processing system, and more particularly, to time-base correction in an image signal processing system.
2. Description of the Related Art
As early as the 1950s, time-base correction of television signals in various forms has been widely applied in the field of video tape recording (VTR). Further, with the evolution of TV technology, the time-base correction techniques are still continuously utilized in various analog and digital TV systems. The time-base correction is generally utilized for reducing short-term and/or long-term timing variations in a TV signal.
In some specific applications, analog TV signal is digitized, e.g. by an analog-to-digital converter at a fixed sampling clock which is asynchronous with respect to the horizontal line frequency of the television signal. However, since time base errors in a TV signal may cause horizontal line periods to be different in length, the digitized TV signal sampled at the fixed sampling clock may thus contain different numbers of samples among various horizontal line periods.
In order to solve the above problem caused by the time-base errors, U.S. Pat. No. 5,600,379 discloses a television digital signal processing system, which is incorporated herein by reference. In the television digital signal processing system, a TV signal is digitally re-sampled by a digital re-sampler after a digital sampling process and then the re-sampled TV signal is stored into a FIFO (First-In First-Out) memory. The television digital signal processing system can control two phase-locked loops by a horizontal synchronization signal (Hsync), such that one of the phase-locked loops can control the sampling-rate-conversion ratio of the digital re-sampler and produce a reset signal for resetting the FIFO memory, and the other phase-locked loop can produce a clock for reading out the re-sampled samples from the FIFO memory.
However, in order to achieve the object, the FIFO memory of the above television digital signal processing system is required to have sufficient memory space, otherwise the FIFO memory may easily encounter an overflow or underflow situation while the re-sampled TV signal within a comparatively short period produces a large number of time-base errors; for example, while the reading process of a VTR is not smooth or the wireless environment for transmitting/receiving the TV signal is dramatically altered.